display a grease pencil object ontop of a 3d model(despite location)
If you mean to make the GP object always be in front of everything, then there is a setting for it. Check out this tutorial video to find out where (because I can't specifically remember where....): 

https://youtu.be/1HRYC4Q1ajI

it's somewhere near the end, like past halfway. it had to do with depth order... may have moved from 2.8 to 2.82.

Hope that helps...